The present invention relates to an oxidative diesel control catalyst which has a high conversion rate for hydrocarbons and carbon monoxide and an inhibited oxidation effect on nitrogen oxides and sulfur dioxide. The catalyst structure contains a monolithic catalyst element with throughflow passages of ceramic or metal coated with an activity-promoting dispersion coating of the fine-particle metal oxides aluminum oxide, titanium oxide, silicon oxide, zeolite or mixtures thereof as support for the catalytically active components. The active components are present in the form of platinum, palladium, rhodium and/or iridium doped with vanadium or in contact with an oxidic vanadium compound.
Diesel exhaust gases contain carbon monoxide, aldehydes, hydrocarbons, polyaromatic hydrocarbons (PAC), sulfur dioxide and nitrogen oxides as pollutants which should be removed by suitable cleaning of the exhaust gases. The sulfur dioxide in the exhaust gas is formed from the sulfur present in a quantity of about 0.3% in the diesel fuel and leads to a sulfur dioxide component in the exhaust gas of 10 to 200 ppm, depending on the particular load and speed of the engine. Although diesel engines produce nitrogen oxides to a far lesser extent than spark-ignition engines, their percentage content in the exhaust gas is still around three times higher than in the exhaust gas of a spark-ignition engine after cleaning with a controlled three-way catalyst.
In addition to these pollutants which are substantially gaseous at the typical exhaust gas temperatures of a diesel engine of 225.degree. to 350.degree. C., diesel engines emit exhaust particles in considerable quantities, depending on the mode of operation. These particles consist of a sooty core and--adsorbed thereon--unburnt hydrocarbons, polyaromatic hydrocarbons (PAC) and also metal compounds, water and sulfates.
The three-way catalysts used in spark-ignition engines cannot be used to control diesel exhaust because diesel exhaust gases have a high oxygen content of 1 to 15 vol-%. This leads to air ratios lambda of the diesel exhaust gases or greater than 1. In contrast, three-way catalysts require stoichiometrically composed exhaust gases with air ratios lambda of 1 for oxidation of the hydrocarbons and carbon monoxide and for simultaneous reduction of the nitrogen oxides.
Filters based on fine-pored ceramic monoliths with reciprocally blocked passages (so-called wall flow filters), foam ceramic, wire mesh packs, ceramic tubes, ceramic fiber wound filters, etc. have already been proposed for reducing the particle emission of diesel exhaust gases containing oxides of sulfur and nitrogen. The diesel exhaust particles can be removed from the exhaust gas stream by means or filtration units such as these. However, the particles filtered off can only be eliminated by burning to regenerate the filters in a few operational states in which the exhaust gas temperature is sufficiently high.
It is known that the regeneration behavior of the filter systems mentioned above can be improved by coating with catalyst substances which reduce the ignition temperature, such as vanadium pentoxide, vanadates, for example AgVO.sub.3, and perrhenates. These active substances may be doped with a fine-particle support material and a noble metal introduced by impregnation, such as platinum, may also be present (see DE-OS 32 32 729, DE-OS 31 41 713 and DE-OS 34 07 712).
It has meanwhile been found that the conversion efficiency of the wall flow filters now predominantly used for hydrocarbons and carbon monoxide is unsatisfactory, particularly at the low exhaust gas temperatures of diesel engines, even if the filters are coated with catalytically active components of the type mentioned above. In addition, the use of wall flow filters coated with catalyst and support materials involves the disadvantage of a high exhaust gas back pressure which impairs engine efficiency, particularly where the exhaust gases contain increased particle levels. Efforts to overcome this disadvantage by increasing the catalyst loading have not been successful. Increasing the geometric dimensions to reduce the back pressure is prevented by the limited space available in most vehicles.
The first major improvement in diesel exhaust control was provided by the diesel oxidation catalyst according to DE-OS 39 40 758. The catalyst in question is a catalyst which operates continuously without particle deposition and cyclic particle elimination for oxidative diesel exhaust control, which has a high conversion rate for hydrocarbons and carbon monoxide at low temperatures and an inhibited oxidation effect on NO and SO.sub.2 and which contains as active component vanadium compounds and platinum group metals applied to fine-particle aluminum oxide, titanium oxide and silicon oxide, zeolite and mixtures thereof as activity-promoting supports.
Compared with wall flow filters, this known catalyst shows an increased conversion rate for the gaseous pollutants combined with the effective elimination of particles by burning. Assuming cell density to be the same, this positive effect can be explained by the fact that the molecules or agglomerates passing through the long passages of the free-throughflow monolith or honeycomb effectively come into contact with the catalytically coated passage surface far more frequently than in the case of wall flow filters. In the latter case, each exhaust gas constituent first passes only once through the catalytically coated porous wall and then enters the middle of the outflow passage through the flow streams entering a given passage through the four adjacent passages, is concentrated and is prevented from entering into wall contacts of comparable extent.
The extent to which diesel exhaust gases can be cleaned with the described diesel oxidation catalyst represent a significant improvement over the cleaning effect obtainable with filter systems. However, in view of the more stringent legal requirements which diesel exhaust control now has to satisfy, there is a need for further reduction in particle emission and for an improvement in long-term stability for the same high conversion rate for the gaseous pollutants hydrocarbons, carbon monoxide and nitrogen oxides.
Since the agglomeration of the exhaust particles is promoted to a very considerable extent by the presence of sulfates in the exhaust gas, measures must be taken to reduce particle emission and to improve suppression of the oxidation of the sulfur present in the diesel fuel from SO.sub.2 to SO.sub.3.